[PATCH v2] s390/rwlock: Add contention tracepoints to rwlock slowpath

[PATCH v2] s390/rwlock: Add contention tracepoints to rwlock slowpath

[Jan Polensky]

Instrument arch_read_lock_wait() and arch_write_lock_wait() with
trace_contention_begin() and trace_contention_end().

These tracepoints are used by lock contention analysis tools such as
perf lock contention to identify contended rwlocks and measure wait
times. The generic implementation in kernel/locking/qrwlock.c already
emits the same events from its read and write slowpaths.

For arch_read_lock_wait(), the in_interrupt() fast-path is intentionally
left outside the tracepoint scope. That path spins without entering the
wait queue, so it does not represent queue-based contention and its
duration is not comparable to the normal slowpath. This matches the
behaviour of queued_read_lock_slowpath() in qrwlock.c.

The include for <trace/events/lock.h> is already present from
commit ffa796cc1f45 ("s390/spinlock: Add contention tracepoints to
lock slowpath").

Signed-off-by: Jan Polensky <japo@linux.ibm.com>
---
v1 -> v2:
- Fix tracepoint placement in arch_read_lock_wait() to avoid recursion deadlocks.

 arch/s390/lib/spinlock.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/s390/lib/spinlock.c b/arch/s390/lib/spinlock.c
index dbabca35c008..c84991a1be20 100644
--- a/arch/s390/lib/spinlock.c
+++ b/arch/s390/lib/spinlock.c
@@ -318,6 +318,7 @@ void arch_read_lock_wait(arch_rwlock_t *rw)

 	/* Remove this reader again to allow recursive read locking */
 	__atomic_add_const(-1, &rw->cnts);
+	trace_contention_begin(rw, LCB_F_SPIN | LCB_F_READ);
 	/* Put the reader into the wait queue */
 	arch_spin_lock(&rw->wait);
 	/* Now add this reader to the count value again */
@@ -326,6 +327,7 @@ void arch_read_lock_wait(arch_rwlock_t *rw)
 	while (READ_ONCE(rw->cnts) & 0x10000)
 		barrier();
 	arch_spin_unlock(&rw->wait);
+	trace_contention_end(rw, 0);
 }
 EXPORT_SYMBOL(arch_read_lock_wait);

@@ -333,6 +335,7 @@ void arch_write_lock_wait(arch_rwlock_t *rw)
 {
 	int old;

+	trace_contention_begin(rw, LCB_F_SPIN | LCB_F_WRITE);
 	/* Add this CPU to the write waiters */
 	__atomic_add(0x20000, &rw->cnts);

@@ -349,6 +352,7 @@ void arch_write_lock_wait(arch_rwlock_t *rw)
 	}

 	arch_spin_unlock(&rw->wait);
+	trace_contention_end(rw, 0);
 }
 EXPORT_SYMBOL(arch_write_lock_wait);

--
2.53.0